The exhaust muffler is one of the most effective methods to control the exhaust noise of diesel generator. In this paper, a new type of impedance composite muffler is designed for the spectral characteristics of diesel engines and the shortcomings of existing mufflers. The main idea of the design is to reasonably design the key structures such as the length-diameter ratio, the insert-pipe, and the sound-absorbing material of the muffler under the condition of keeping the shape unchanged and low cost, so as to achieve the broadband noise reduction performance and improve the working efficiency of the diesel engine.